Miscellanea : Ecological factors limiting the extension of malaria in the Southwest Pacific : their bearing on malaria control or eradication programmes

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(1)Miscellanea : Ecological factors limiting the extension of malaria in the Southwest Pacific : their bearing on malaria control or eradication programmes Autor(en):. Peters, W.. Objekttyp:. Article. Zeitschrift:. Acta Tropica. Band (Jahr): 22 (1965) Heft 1. PDF erstellt am:. 28.01.2022. Persistenter Link: http://doi.org/10.5169/seals-311260. Nutzungsbedingungen Die ETH-Bibliothek ist Anbieterin der digitalisierten Zeitschriften. Sie besitzt keine Urheberrechte an den Inhalten der Zeitschriften. Die Rechte liegen in der Regel bei den Herausgebern. Die auf der Plattform e-periodica veröffentlichten Dokumente stehen für nicht-kommerzielle Zwecke in Lehre und Forschung sowie für die private Nutzung frei zur Verfügung. Einzelne Dateien oder Ausdrucke aus diesem Angebot können zusammen mit diesen Nutzungsbedingungen und den korrekten Herkunftsbezeichnungen weitergegeben werden. Das Veröffentlichen von Bildern in Print- und Online-Publikationen ist nur mit vorheriger Genehmigung der Rechteinhaber erlaubt. Die systematische Speicherung von Teilen des elektronischen Angebots auf anderen Servern bedarf ebenfalls des schriftlichen Einverständnisses der Rechteinhaber. Haftungsausschluss Alle Angaben erfolgen ohne Gewähr für Vollständigkeit oder Richtigkeit. Es wird keine Haftung übernommen für Schäden durch die Verwendung von Informationen aus diesem Online-Angebot oder durch das Fehlen von Informationen. Dies gilt auch für Inhalte Dritter, die über dieses Angebot zugänglich sind.. Ein Dienst der ETH-Bibliothek ETH Zürich, Rämistrasse 101, 8092 Zürich, Schweiz, www.library.ethz.ch http://www.e-periodica.ch.

(2) Ecological Factors Limiting the Extension of Malaria in the Southwest Pacific—Their Bearing on Malaria Control or Eradication Programmes. By W. Peters Lately Senior Malariologist, Port Moresby *.. /. Introduction. Belatively little attention has been devoted to examining closely those natural factors that have impeded the geographical extension of malaria in such areas as the Southwest Pacific. While in some localities such as the Northern Territory of Australia malaria has regressed spontaneously (Black, 1950), in various islands in the eastern part of this region it appears to be more prevalent, for example on Rennell Island (Laird and Laird, 1959) and Sikaiana (Black, 1952). In the highlands of the main island of New Guinea too, malaria may be extending into new areas (Peters et al., 1958). The recent programmes of residual insecticide spraying, sometimes combined with mass chemoprophylaxis or therapy have not fulfilled their earlier promise (Peters, 1962). In this paper an attempt is made to examine the situation from the relatively neglected perspective of the natural, ecological factors that limit the spread of malaria, in the hope that new light may be shed on various aspects of which advantage may be taken.. 2.. The limits of distribution of malaria and its vectors in the Southwest Pacific.. The distribution of malaria is shown in figure 1. Within this area exist small localities, such as parts of the mountainous central region of the main island of New Guinea, from which malaria is absent although Anophelism without malaria occurs in places. The main malaria vectors are summarised in table 1. Although as regards its behaviour, A. punctulatus is the most dangerous vector, A. farauti undoubtedly poses the most serious control problems in this region. In view of the importance for modern control methods of even slight differences in breeding, resting and feeding habits it is essential to call attention again to the need for clarification of the differential systematics and bionomics of this vector complex. It can be said in general that malaria is holoendemic (sensu Metselaar and Van Thiel, 1959) and stable (sensu MacDonald, 1957) in most coastal and subcoastal areas of the New Guinea mainland where A. punctulatus is the dominant species such as Maprik in the Sepik District (Peters and Standfast, 1960). In some areas where A. farauti may be the sole vector, it transmits malaria intensely and produces an unstable pattern varying from hypo- to * Present address: Research Laboratories. of CIBA Ltd., Basle, Switzerland.. of the Pharmaceutical. Department.

(3) Peters, Ecological Factors Limiting the Extension of Malaria. K. y. 0. A. »ALARI*. y. a«s ». III. \. 63. FREE. \^. m. «u MALARIA. FREE. malaria free. ^. Distribution or A.farauti Distribution of A. punctulatus. Fig.. Approximate distribution of malaria, Anopheles farauti and A. punctulatus in the Southwest Pacific. (Note that the distribution of A. koliensis closely follows that of A. punctulatus.). 1.. holoendemicity. With increasing altitude inland the picture on the New Guinea mainland changes. In the mountainous area between 1500 and 2000 metres, A. farauti, aided in particularly favourable years by localised breeding of A. punctulatus or A. koliensis, produces severe epidemics (Metselaar, 1959; Peters and Christian, 1960). In Australia there may remain a few residual endemic malaria foci but Anophelism without malaria now exists in the area indicated in figure 1.. 3.. Main factors in the equation of malaria transmission.. The three factors in this cycle, the parasite, the mosquito and man have been correlated mathematically by MacDonald (1957).. —rlogep a2b pn. „. "r" and "b" refer to the duration of infection in the human host and his degree of premunity respectively, "a" and "p" are directly related to the vector, "a" representing its biting frequency on man and "p" its natural longevity. "n" is the duration of the extrinsic cycle of the malaria parasite in the vector and differs for the different species of Plasmodium, other conditions being equal. "m" is the adult mosquito population density above which an increase in the quantum of malaria in a community can occur. *.

(4) Acta Trop. XXII,. 64. 1, 1965. — Miscellanea. TABLE 1. Malaria vectors of the Southwest Pacific. Subgenus. Major vectors of widespread distribution. Proved minor vectors of limited distribution. bancroftii. Anopheles (Anopheles). Suspected but unproven. vectors. Giles, 1902. pseudobarbirostris Ludlow, 1902. Anopheles. farauti. karwari. subpictus. (Cellia). Laveran, 1902. James, 1903. Grassi, 1899. punctulatus Dönitz, 1901. Lee & Woodhill,. koliensis Owen, 1945. amictus 1944. hilli. annulipes. Walker, 1856 amictus amictus Edwards, 1921. longirostris Brug, 1928 meraukensis Venhuis, 1932. Data on the various factors in this equation as they relate to Anopheline ecology and the intensity of malaria transmission in the marginal areas of its distribution are completely lacking except for part of the highlands of the Territory of Papua and New Guinea (Peters and Christian, 1960), where "m" varied between 11.2 in the dry and 0.67 in the rainy, epidemic season. 4.. Vectorial capacitg of the main vectors.. The vectorial capacity of the main vectors in a given area is governed by ecological conditions that may influence "a", "p" or "n".. Natural longevity ("p"). All three species of the A. punctulatus complex are long-lived and, from this point of view, potentially efficient vectors. A. farauti has an even greater longevity in the highlands, at least during the epidemic season, than in a). coastal or subcoastal areas. The relative humidity undoubtedly plays a large role in the variations of longevity of the vectors and, in turn, is dependent upon the total rainfall. In the highlands more efficient drainage, a greater evaporation rate and the more seasonal character of the precipitation result in a distinct dry season when the macroclimate is unfavourable for Anopheline survival. It also seems likely that the increase in length of the gonotrophic cycle observed by Slooff (1961) in A. koliensis when oviposition sites were scarce takes place in A. farauti under similar circumstances. Summing up, we may say that in the highlands, as a result of seasonal variations in longevity, an anti-Anopheline residual spraying campaign conducted in the dry season stands a better chance of success than in the rainy season. A 12% increase in the daily mortality of A. farauti would theoretically interrupt malaria transmission in the dry season but a 20% increase would be needed in the wetter months (Peters and Christian, 1960)..

(5) Peters, Ecological Factors Limiting the Extension of Malaria. 65. TABLE 2. The longevity of Pacific malaria vectors.. "p". Author. Species. Locality. A. punctulatus. Nimboran Valley. 0.79. Metselaar, 1957. Maprik. 0.87. Peters & Stand¬ fast, 1960. A.. farauti. 0.90 ± 0.2 Peters & Christian, 1960. Minj Nimboran Valley. 0.81. ?0.87. Maprik Merauke. 0.68. Peters & Standfast. Van den Assem,. Fergusson Island (D'Entrecastaux). 0.85. Nissan Island. 0.84. Spencer, 1961. Nimboran Valley. 0.79. Metselaar. Hollandia. A. bancroftii sensu lata. b). Insecticide spraying in progress. Spencer & Spencer, 1960. 0.74-0.91. ?0.87. Maprik. 0.92-0.88 depending on season. Metselaar. 1959. A. koliensis. Remarks. Van den Assem Peters & Standfast. Kimaan (Frederik Hendrik Island). 0.64. Van den Assem. Merauke. 0.69. Van den Assem. Insecticide spray¬ ing in progress. Anthropophilic habits ("a").. A. farauti is readily deviated from man to non-human hosts. Nevertheless, when man is the most readily accessible host, the anthropophilic index is high. A. punctulatus is more inherently anthropophilic. An unusual feature of most of the Southwest Pacific islands, including New Guinea itself, is the relative scarcity of domestic animals living in close association with man. Curiously enough the level of endemicity of malaria is low in Iwo of the most swampy areas of the New Guinea mainland although A. farauti is present at least in small numbers (van Dijk, 1958, Bamgi-Ia area; Peters, 1957, Fly River). In these areas the human population is very thinly spread whereas there is a relatively large wild fauna so that the ratio of availability of human to animal hosts is low. It has been observed that A. farauti enters houses to feed at Minj less readily in the dry season. Since, in the cold evenings, the indigenous population 6. Acta Tropica 22,. 1, 1965.

(6) Acta Trop. XXII,. 66. 1,. 1965. — Miscellanea. tends to retire indoors early, there is possibly a reduced man-mosquito contact which assists in reducing the amount of transmission during the dry months. Important in relation to the natural limitation of transmission in the highlands is the fact that A. annulipes, which is the dominant species as judged by larval population densities, is very rarely taken indoors or in window traps and has never been recorded as biting man in New Guinea (Peters and Christian, 1963).. Inherent receptivity to Plasmodia. Unfortunately, the species with the widest distribution, A. farauti, is both long-lived and a good host for Plasmodia. It may prove of considerable interest to study the inheritance of receptivity to malaria parasites in such a vector. Ward (1963) has recently investigated the receptivity of Aëdes aegypti for P. gallinaceum and selectively bred a 98% resistant strain. c). Anopheline population dgnamics ("n"). The size of the adult vector population is the product of the chances of survival of the egg, larval instars and pupa. These in turn depend upon the following interrelated ecological conditions. d). (i) Selection of oviposition sites.. All authors agree that A. farauti is the most adaptable vector as regards type of oviposition site and that it can develop in almost any type of surface water or water in natural or artificial containers when the necessity arises. Its ability to develop completely in brackish water has undoubtedly facilitated its extensive spread to the east over widely separated coralline-island chains. A. koliensis too can adopt many types of breeding sites except those containing significant quantities of sea water. In contrast the typical A. punctulatus breeding site, small transient puddles devoid of vegetation in clay-bearing soil, exposed to full sunlight, is almost obligatory for this species, a factor that accounts largely for its limited and patchy distribution and relative inability to extend its domain.. (ii) Availability of breeding sites. It is quite clear that, particularly in their marginal areas of distribution, the natural limitation of available breeding sites for the various species has played a large part in creating these margins. The limitations are created by. Topography Soil type Total rainfall Daily rainfall pattern. Good natural drainage or highly porous subsoils (e.g. coralline) prevent the accumulation of suitable collections of surface water. It should be remembered however that any unusual disturbance of the soil surface in an area, such as the clearing of jungle, road building or diverse wartime activities leads to the production of unlimited breeding sites for A. punctulatus, not to mention A. farauti and A. koliensis. Of greater importance in limiting Anopheline breeding in view of their preferential breeding sites is the daily rainfall pattern. The significance of this was intimated by Laird (1946) whose observations, together with our own (Peters and Christian, 1963), fully support the suggestion of Christie (1959) that daily rainfall patterns determine the size of adult population densities of A. punctulatus as they do A. gambiae in Africa..

(7) Peters, Ecological Factors Limiting the Extension of Malaria. 67. (iii) Larval competition. The writer has observed that on certain atolls, such as Tasman Island, breeding sites are at such a premium that enormous numbers of Culex annulirostris occur in them and it seems most likely that cannibalism must take place. This may well play a role in limiting the breeding of A. farauti on those isolated coralline atolls where the two species coincide or where the latter is introduced from time to time. In the New Guinea highlands Peters and Christian (1963) have suggested that larval competition between A. annulipes and A. farauti must play an important part in keeping down the numbers of the latter. In the subcoastal area of Maprik they found that Culicine larval densities rose greatly when the competition pressure previously exerted by A. punctulatus was relieved as the adult Anophelines succumbed to residual house spraying. (iv) Parasites and predators.. The relation between Anopheline larvae, their endo- and exoparasites and predators has been admirably documented by Laird (1947, 1956, 1959). As a result of his extensive investigations he recommended that the possibilities of biological control should be further investigated. 5. Geographical isolation.. As the. flight range of. farauti and A. punctulatus lies between Va and mile (0.8-1.6 km), one is inclined to look to various methods of passive transport in order to explain the extensive distribution of A. farauti to the east. That exotic species can be transported by human agency appears very likely, especially if the larval stages can withstand exposure to saline water (e.g. in canoes) or the adults are particularly hardy. Belkin (1962) has summarised the methods by which transport across sea barriers may occur. A.. 1. Laird. (1956) emphasised that Buxton's Line at 170° E latitude merely marks the present eastern limit of the dispersal of A. farauti and that "it cannot be claimed that any general factor other than isolation is operating to prevent the extension of the range of A. farauti to those tropical islands east of Buxton's. line where the requisite ecological conditions present themselves". 6.. Relation to malaria control or eradication programmes.. From the above account it is obvious that, while various factors operate to circumscribe the distribution of malaria in the Southwest Pacific, such factors when varied may contribute to an extension of the disease, either. quantitatively or geographically. There is little doubt that much ground can still be gained by the judicious application of suitable insecticides and anti-malarial drugs in the combating of malaria in this region. Nevertheless the indications are that these alone may not suffice completely to interrupt malaria transmission. Various suggestions made above can be applied with little question. These are: 1. Quarantine measures to avoid artificial dispersal of vectors to new localities. The provision of larger domestic animals (cattle, pigs) in special stables which are suitable for residual insecticide spraying. 3. Careful avoidance of the production of additional breeding sites. 4. Construction and maintenance of proper drainage and/or irrigation systems where practical. 5. The stabilising of migrant populations in suitable native-style housing. 2..

(8) Acta Trop. XXII,. 68. 1, 1965. — Miscellanea. Other lines of attack require careful prior scientific enquiry and include: 1. Ecological studies to clarify the importance of larval competition and its possible practical application. 2. Ecological investigations into the natural parasites and predators of all Anopheline stages and exploration of their potential value for biological control of Anopheline vectors. 3. Systematic studies designed to clarify behavioural differences and taxonomic status of closely related vectors. 4. Introduction on a trial basis of such predators as suitable species of. "annual fish". Of more general in addition to local interest would be: 1. Exploration of the morphological and genetic basis of vector susceptibility to Plasmodial infection. 2. The development of genetically selected insusceptible strains. 3. The selection of sterile mutant males for admixture with local populations of the same taxon. 4. Further ecological studies of malaria in the marginal areas of its distribution to seek so far undetermined limiting factors. 7.. Acknowledgments.. My thanks are due to all my old colleagues of the Department of Public Health, Territory of Papua and New Guinea and others who devoted years of effort in the Malaria Control Service of the late Netherlands New Guinea.. 8.. References.. Assem, J. van den. (1959). Daily mortality in four species of New Guinea Anophelines. — Trop, geogr. Med. 11, 223-326 Belkin, J. N. (1962). The mosquitoes of the South Pacific (Diptera, Culicidae), vols. I, II. — Berkeley: Univ. California Press. Black, R. H. (1950). Anophelism without malaria in Northern Australia: a malaria survey of part of the Northern Territory and the East Kimberleys District. — Ann. trop. Med. Parasit. 44, 207-211. Black, R. H. (1952). Malaria in the British Solomon Islands Protectorate. Techn. Pap. No. 33, pp. 1-31. — Nouméa: S. Pacific Commission. Christie, M. (1959). A critical review of the role of the immature stages of Anopheline mosquitoes in the regulation of adult numbers, with particular reference to Anopheles gambiae. — Trop. Dis. Bull. 56, 385-399. Dijk, W. J. O. M. van (1958). Transmission of Wuchereria bancrofti in Nether¬ lands New-Guinea. — Trop, geogr. Med. 10, 21-33. Laird, M. (1946). Observations on Anopheles punctulatus Dönitz, 1901, and Anopheles farauti Laveran, 1902, at Palmalmal and Manginuna, New Britain, during July and August 1945. — Trans, roy. Soc. N. Zealand 76, 148-157. Laird, M. (1947). Some natural enemies of mosquitoes in the vicinity of Palmalmal, New Britain. — Trans, roy. Soc. N. Zealand 76, 453-476. Laird, M. (1956). Studies of mosquitoes and freshwater ecology in the South Pacific. — R. Soc. N. Zealand, Bull. No. 6 (Wellington), pp. 1-213. Laird, M. (1959). Fungal parasites of mosquito larvae from the Oriental and Australian regions, with a key to the genus Coelomyces (Blastocladiales: Coelomycetaceae). — Canad. J. Zool. 37, 781-791..

(9) Peters, Ecological Factors Limiting the Extension of Malaria. 69. Laird,. M. & Laird, E. (1959). Chapter 30 Culicidae and Haematozoa from Bellona and Rennell. In: The Natural History of Rennell Island, British Solomon Islands, v. 2 (Invertebrates, pars), pp. 213-234, pl. 1. — Copenhagen:. Torben Wolff. MacDonald, G. (1957). The epidemiology and control of malaria, pp. 1-201. — London: Oxford Univ. Press. Metselaar. D. (1957). A pilot projet of residual insecticide spraying in Nether¬ lands New Guinea, contribution to the knowledge of holo-endemic malaria. — Thesis, University of Leyden, pp. 1-128. Metselaar, D. (1959). Two malaria surveys in the central mountains of Netherlands New Guinea. — Amer. J. trop. Med. Hyg. 8, 364-367. Metselaar, D. & Thiel, P. H. van. (1959). Classification of malaria. — Trop. geogr. Med. 11, 157-161. Peters, W. (1957). A malaria survey in the Western District of Papua. — Papua N. Guinea med. J. 2, 25-38. Peters, W. (1962). A critical survey of the results of malaria eradication and control programmes in the South-West Pacific. — Ann. trop. Med. Parasit. 56, 20-32. Peters, W. & Christian, S. H. (1960). Studies on the epidemiology of malaria in New Guinea. Pt. IV: Unstable highland malaria—the clinical picture. Pt. V: Unstable highland malaria—the entomological picture. Pt. VI:. highland malaria—analysis of data and possibilities for eradication of malaria. — Trans, roy. Soc. trop. Med. Hyg. 54, 529-548. Peters, W., Christian, S. H. & Jameson, J. L. (1958). Malaria in the highlands of Papua and New Guinea. — Med. J. Aust. 2, 409-416 Peters, W. & Christian, S. H. (1963). The bionomics, ecology and distri¬ bution of some mosquitoes (Diptera: Culicidae) in the Territory of Papua and New Guinea. — Acta trop. 20, 35-79. Peters, W. & Standfast, H. A. (1960). Studies on the epidemiology of malaria in New Guinea. Pt. I: Holoendemic malaria—the clinical picture. Pt. II: Holoendemic malaria—the entomological picture. Pt. Ill: Holoendemic malaria—analysis of data and relation to control. — Trans, roy. Soc. trop. Unstable. Med. Hyg. 54, 242-260. Slooff, R. (1961). Field observations on the biting activity of Anopheles kolien¬ sis Owen. — Trop, geogr. Med. 13, 67-76. Spencer, M. (1961). Malaria control-pre-operational entomological report,. April. 1960.. Spencer, T. & Spencer, M. (1960). Malaria assessment methods. — Papua N. Guinea med. J. 4, 55-61. Ward, R. A. (1963). Genetic aspects of the susceptibility of mosquitoes to malarial infection. — Exp. Parasit. 13, 328-341..

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